CS9222 ADVANCED OPERATING SYSTEMS

CS9222 Advanced Operating
System
Unit – II
Dr.A.Kathirvel
Professor & Head/IT - VCEW

Unit - II
Introduction – Issues – Communication Primitives –
Inherent Limitations - Lamport’s Logical Clock; Vector
Clock; Causal Ordering; Global State; Cuts;
Termination Detection. Distributed Mutual Exclusion
– Non-Token Based Algorithms – Lamport’s
Algorithm - Token-Based Algorithms – Suzuki-
Kasami’s Broadcast Algorithm – Distributed Deadlock
Detection – Issues – Centralized Deadlock-Detection
Algorithms - Distributed Deadlock-Detection
Algorithms. Agreement Protocols – Classification -
Solutions –Applications.

What is Distributed Systems?
Distributed System is used to describe a
system with the following characteristics:
Consists of several computers that do not
share a memory or a clock;
The computers communicate with each other
by exchanging messages over a
communication network; and
Each computer has its own memory and runs
its own operating system.

Architecture of Distributed OS

What is Distributed Operating Systems?
It extends the concepts of resource management and
user friendly interface for shared memory computers a
step further, encompassing a distributed computing
system consisting of several autonomous computers
connected by a communicating network.
A distributed OS is one that looks to its users like an
centralized OS but runs on multiple, independent CPUs.
The key concept is transparency. In other words, the use
of multiple processors should be invisible to the user

Issues in Distributed OS
Global Knowledge
Naming
Scalability
Compatibility
Process Synchronization
Resource Management
Security
Structuring

Global Knowledge
No Global Memory
No Global Clock
Unpredictable Message Delays

Naming
Name refers to objects [ Files, Computers etc]
Name Service Maps logical name to physical
address
Techniques
LookUp Tables [Directories]
Algorithmic
Combination of above two

Scalability
Grow with time
Scaling Dimensions – Size, Geographically &
Administratively
Techniques – Hiding Communication
Latencies, Distribution & Caching

Scaling Techniques (1)
Hide Communication Latencies
1.4
The difference between letting:
a) a server or
b) a client check forms as they are being filled Scalability cont….

Distribution
1.5
An example of dividing the DNS name space into zones.
Scalability cont….

Replication
Replicate components across the distributed
system
Replication increases availability , balancing
load distribution
Consistency problem has to be handled

Compatibility
Interoperability among resources in system
Levels of Compatibility – Binary Level,
Execution Level & Protocol Level

Process Synchronization
Difficult because of unavailability of shared
memory
Mutual Exclusion Problem

Resource Management
Make both local and remote resources
available
Specific Location of resource should be hidden
from user
Techniques
Data Migration [DFS, DSM]
Computation Migration [RPC]
Distributed Scheduling [ Load Balancing]

Security
Authentication – a entity is what it claims to
be
Authorization – what privileges an entity has
and making only those privileges available

Structuring
Techniques
Monolithic Kernel
Collective Kernel [Microkernel based , Mach, V-
Kernel, Chorus and Galaxy]
Object Oriented OS [Services are implemented as
objects, Eden, Choices, x-kernel, Medusa, Clouds,
Amoeba & Muse]
Client-Server Computing Model [Processes are
categorized as servers and clients]

Communication Primitives
High level constructs [Helps the program in
using underlying communication network]
Two Types of Communication Models
Message passing
Remote Procedure Calls

Message Passing
Two basic communication primitives
SEND(a,b) , a Message , b Destination
RECEIVE(c,d), c Source , d Buffer for storing the
message
Client-Server Computation Model
Client sends Message to server and waits
Server replies after computation
Message Passing cont..

Design Issues
 Blocking vs Non blocking primitives
 Nonblocking
 SEND primitive return the control to the user process as soon as the
message is copied from user buffer to kernel buffer
 Advantage : Programs have maximum flexibility in performing
computation and communication in any order
 Drawback  Programming becomes tricky and difficult
 Blocking
 SEND primitive does not return the control to the user process until
message has been sent or acknowledgement has been received
 Advantage : Program’s behavior is predictable
 Drawback  Lack of flexibility in programming
Message Passing cont..

Design Issues cont..
 Synchronous vs Asynchronous Primitives
 Synchronous
 SEND primitive is blocked until corresponding RECEIVE primitive
is executed at the target computer
 Asynchronous
 Messages are buffered
 SEND primitive does not block even if there is no corresponding
execution of the RECEIVE primitive
 The corresponding RECEIVE primitive can be either blocking or
non-blocking

Details to be handled in Message Passing
Pairing of Response with Requests
Data Representation
Sender should know the address of Remote
machine
Communication and System failures

Remote Procedure Call (RPC)
RPC is an interaction between a client and a
server
Client invokes procedure on sever
Server executes the procedure and pass the
result back to client
Calling process is suspended and proceeds
only after getting the result from server

RPC Design issues
Structure
Binding
Parameter and Result Passing
Error handling, semantics and Correctness

Structure
RPC mechanism is based upon stub procedures.
Unpack
Execute
Procedure
Stub
Procedure
Remote
Procedure
Return
Pack
result
Local
Procedure
Call
Local
Procedure
Call
User
Program
Stub
Procedure
Pack
parameters
& Transmit
wait
Unpack
Result
Return
from local
call
Client Machine
Server Machine

Binding
Determines remote procedure and machine
on which it will be executed
Check compatibility of the parameters passed
Use Binding Server

Binding
Binding Server
Receive
Query
Return
Server
Address Unpack
Stub
Procedure
Remote
Procedure
Return
Pack
result
Local
Procedure
Call
Local
Procedure
Call
Binding
Server
User
Program
Stub
Procedure
Pack
parameters
& Transmit
wait
Unpack
Result
Return
from local
call
Client Machine
Server Machine
Registering
Services
1
2
3
4
5
6
7
8

Parameter and Result Passing
 Stub Procedures Convert Parameters & Result to appropriate
form
 Pack parameters into a buffer
 Receiver Stub Unpacks the parameters
 Expensive if done on every call
 Send Parameters along with code that helps to identify format
so that receiver can do conversion
 Alternatively Each data type may have a standard format.
Sender will convert data to standard format and receiver will
convert from standard format to its local representation
 Passing Parameters by Reference

Error handling, Semantics and Correctness
 RPC may fail either due to computer or communication failure
 If the remote server is slow the program invoking remote
procedure may call it twice.
 If client crashes after sending RPC message
 If client recovers quickly after crash and reissues RPC
 Orphan Execution of Remote procedures
 RPC Semantics
 At least once semantics
 Exactly Once
 At most once

Correctness Condition
Given by Panzieri & Srivastava
Let Ci denote call made by machine & Wi
represents corresponding computation
If C2 happened after C1 ( C1  C2) &
Computations W2 & W1 share the same data,
then to be correct in the presence of failures
RPC should satisfy
 C1  C2 implies W1  W2

CS9222 ADVANCED OPERATING SYSTEMS

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